What reference ranges mean

They can mean lots of things and you need to know exactly what any reference ranges used actually reflect.

Most blood tests are performed with commercial kits. The manufacturer of each kit is responsible for advising the appropriate ranges of results.

Typically they will have performed the test on a number of supposedly "healthy" people to determine the acceptable range of results. The people concerned may have been admitted to hospital for elective (non urgent) surgery, or worked for the company concerned or been blood donors. They hopefully would have been in the FAQ category re. their health.

They probably didn't belong to any particular ethnic group, and may not have included people at extremes of age, pregnant etc.

Many variables may be such that the reference range quoted is not even slightly relevant to you in particular.

These variables are in part responsible for some tests having very broad ranges quoted.

An example is vitamin B12 levels. My labs have different kits, so slightly different ranges. Both are very wide ranges, in SI units from 150 to 700. What is healthy? I'm pretty happy if the result is over 400, but don't rely on this alone even then. I always have a homocysteine measurement as well, to double check.

Homocysteine is quoted at about 3 to 14 (SI) but I'm unhappy if it is over 8. There are a lot of people walking around with higher homocysteine levels due to subclinical vitamin B12 deficiency. They have obviously been included in the reference range determination.

You are your best "control." Compare tests with previous results.

I would advise everyone to get copies of any blood tests you have. You need a photocopy or printout, as the same test may be done using different kits at different times and in different laboratories. The ranges quoted may be different, so the value of the test by itself is not sufficient.

It will save any future doctor a lot of time and trouble if you are able to provide this information anytime the same test is repeated.

Sometimes the gradual change in a test over time, is a vital clue. An example of this is the MCV.

Your red blood cells live for about 100 days, and become progressively smaller as iron deficiency develops, larger as vitamin B12 or folic acid become deficient.

This can happen without the values leaving the quoted range.

Mrs. C was an example of this. Her MCV had risen progressively over 8 years, initially 82 then 87, 89 and 90. Her homocysteine rose also, from 5.9 to 7.3 over the last 4 years. At the last visit, B 12 was mid-range at 383 (150-700) and red cell folate barely OK at 700 (>630.)
Without the changes in MCV and Hcy, the lowish folate would have been accepted.

Then along came Sam Queen

This guy is the brains behind Designed2Win - previously the Institute for Health Realities. He has a world of experience and a very incisive mind. His reference ranges ae nearly all a bit different and sometimes strikingly so, as they are from people who have been brought to optimal health rather than average health.

He quotes optimum glucose, for instance, as 75-90mg/dL compared with a common range accepted as 60-115. (SI units 4-5 instead of approximately 3-6.)
Another example, important because iron is potentially very toxic, is transferrin saturation, is optimum 30-30% compared to often quoted 15-50%.
As discussed on the thyroid medicine page, his reference range for TSH is also 1.5-2.5 rather than often advised as 0.35-6.0.

Statistical stuff

Any test method will be subject to a certain amount of random error³. Any person will have a certain amount of change in chemistry from one day to another.

On any one occasion these variations may be at their peak and in the same direction, so as to, say, reduce the result of the test.

If the same test is repeated on another day, the chance that the variables will be maximal and in the same direction, is pretty unlikely.

The result will therefore be closer to average. This is called "regression to the mean." Any extreme result is likely to be closer to average the next time it is repeated.

The ranges quoted, assuming equal spread above and below the average, usually¹ include 95% of the reference population². If you have a lot of tests the chance that one of them will be outside this range, increases. If you had 20 tests, that chance would be 64%.

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Notes and references

1. Some results vary greatly between individuals, so may be supplied with restricted reference ranges, such as mean plus 1 standard deviation for serum insulin - only 68% of the reference population.

2. This is the average +/- two standard deviations, in a Gaussian distribution (the classical bell-shaped curve, with most items near the average, progresively fewer the further away from it in either direction.)

3. In the MJA Oct 26th p 737 1968, Dr. J. Grimley Evans mb mrcp wrote an article "The Normal Range: Background, Uses and Limitations".
He wrote

"How many clinical laboratories as a routine print their standard error of the method on their report forms? How many clinicains would know what it meant if they did?"

They didn't and wouldn't. This still is true today, 50 years later. At least they call it a reference range now.
He continued...

"Traditionally, the method of presentation (of the population data used to give a reference range) is to pretend that the distribution of the variable has a particular form (which it never has)- the so-called "normal" or Gaussian curve. The "normal range" is then expressed as the mean plus or minus two standard deviations, so that, in theory, only 2.5% of the population will have values above the range, and 2.5% below. It would be more accurate and more useful to express the results in percentiles, as described by Herrera (1958), on a cumulative distribution graph, which requires no assumptions about the underlying distribution."

This would be wonderful. People wouldn't have the magic upper and lower figures to misinterpret, and would be forced to think about where in the "range" this result sits.

Fast forward to 2017 and the RCPA have just issued a position statement on thyroid function testing which includes...
Subclinical hypothyroidism: Asymptomatic condition where the individual has free T4 within the reference interval and TSH above the laboratory upper reference limit, in the absence of intercurrent illness.

They are in effect saying "inside the reference range is normal,, outside is not."


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